1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to multi-piece golf balls having a core, a mid layer and a cover.
2. Description of the Related Art
Top requirement for golf balls by golf players is their flight performances. The golf players place great importance on flight performances achieved upon shots with a driver, a long iron and a middle iron. The flight performances correlate with resilience performances of the golf ball. Hitting of a golf ball that is excellent in resilience performance leads to a high-speed flight, whereby a great flight distance is attained.
For attaining a great flight distance, an appropriate trajectory height is required. The trajectory height varies depending on the spin rate and launch angle. Golf balls which achieve a high trajectory due to a high spin rate are accompanied by insufficient flight distance. Golf balls which achieve a high trajectory due to a great launch angle can attain a great flight distance. By employing a core having an outer-hard/inner-soft structure, a low spin rate and a great launch angle can be both achieved.
Golf players place great importance also on spin performances of golf balls. A great back spin rate results in small run. For golf players, golf balls which are liable to be spun backwards are apt to be rendered to stop at a target position. Great side spin rate results in easily curved trajectory of the golf ball. For golf players, golf balls which are liable to be spun sidewise are apt to allow their trajectory to curve intentionally. The golf balls that can be easily spun are excellent in control performances. High-level golf players particularly place great importance on control performances upon shots with a short iron.
In light of achievement of various performances, golf balls having a multilayer structure have been proposed. Japanese Unexamined Patent Application, Publication No. H10-328326 (equivalent to U.S. Pat. No. 6,468,169) discloses a golf ball having an inner sphere, an enclosure layer, an inner cover and an outer cover. Japanese Unexamined Patent Application, Publication No. 2001-17575 (equivalent to U.S. Pat. No. 6,271,296) discloses a golf ball having a core, an envelope layer, a mid layer and a cover. Japanese Unexamined Patent Application, Publication No. 2002-272880 (equivalent to US 2001/0024982) discloses a golf ball having a core and a cover. This core is composed of a center and an outer core layer. The cover is composed of an inner cover layer and an outer cover layer. Japanese Unexamined Patent Application, Publication No. 2003-205052 (equivalent to US 2003/0166422) discloses a golf ball having a center, a mid layer and a cover. Japanese Unexamined Patent Application, Publication No. 2004-130072 (equivalent to US 2004/0029648) discloses a golf ball having a core and a cover. This core has a three-layer structure.
When a core having an outer-hard/inner-soft structure and having an excessively large hardness distribution is hit with a driver, great energy loss occurs at this core. The energy loss results in deterioration of the resilience performance. When a core having an outer-hard/inner-soft structure and having an excessively large hardness distribution is hit with a short iron, a low spin rate is achieved. The low spin rate results in deterioration of the control performance.
An object of the present invention is to provide a golf ball that can attain a great flight distance upon hitting with a driver, and that is excellent in a control performance achieved upon hitting with a short iron.